继续获得堆栈空异常

Question

我正在创建一个RPN计算器，其中添加了某些执行某项任务的运算符。程序运行并进行简单的加法，减法，乘法和除法，但是当我输入运算符“p”或“n”时，我得到一个堆栈空异常。当我在操作员面前明确输入两个整数时，我无法弄清楚为什么我会得到这个。此外，当我使用任何其他运算符，如“r”或“p”时，我得到0的结果，这是不应该发生的。

import java.util.*;

public class RPNCalculator {
    public static int op1;
    public static int op2;
    public static Stack<Integer> stack;

    public RPNCalculator()
    {
        stack = new Stack<Integer>();
    }

    public static int performCalculation(String expression)
    {
        int op1, op2, result = 0;
        String token;
        Scanner parser = new Scanner(expression);

        while (parser.hasNext())
        {
            token = parser.next();

            if (isOperator(token))
            {
                op2 = (stack.pop()).intValue();
                op1 = (stack.pop()).intValue();
                result = evaluateOperator(token.charAt(0), op1, op2);
                stack.push(new Integer(result));
            }
            else
                stack.push(new Integer(Integer.parseInt(token)));
        }

        return result;
    }

    private static boolean isOperator(String token)
       {
           return ( token.equals("+") || token.equals("-") ||
                    token.equals("*") || token.equals("/") ||
                    token.equals("%") || token.equals("m") ||     
                    token.equals("r") || token.equals("d") ||
                    token.equals("p") || token.equals("n") ||
                    token.equals("f") || token.equals("c") ||
                    token.equals("q") || token.equals("h") ||
                    token.equals("?") );
       }

    public static int evaluateOperator(char operator, int op1, int op2)
    {
        int result = 0;
        int top = 0;
        switch (operator)
        {
            case '+':
                result = op1 + op2;
                break;
            case '-':
                result = op1 - op2;
                break;
            case '*':
                result = op1 * op2;
                break;
            case '/':
                result = op1 / op2;
                break;
            case '%':
                result = op2 / op1;
                break;
            case 'm':
                result = op2 * -1;
                break;
            case 'r': op1 = op2;
                      op2 = op1; 
                      stack.push(op1);
                      stack.push(op2);
                break;  

            case 'd': stack.push(op2);
                break;

            case 'p': top = stack.peek();
                      System.out.println(top);
                break;

            case 'n': top = stack.peek();
                      result = top;
                      stack.pop();
                break;

            case 'f': System.out.print(stack);
                break;

            case 'c': stack.clear();                            
                break;

            case 'q': System.exit(1);
                break;

            case 'h': help();
                break;
            case '?': help();
                break;

        }
        return result;
    }


    public static void help()
    {
      System.out.println("p print top");
      System.out.println("n print top and remove");
      System.out.println("d duplicate top");
      System.out.println("r exchange top two items");
      System.out.println("f print contents of stack");
      System.out.println("c clear stack");
      System.out.println("+ add");
      System.out.println("- subtract");
      System.out.println("* multiply");
      System.out.println("/ integer divide");
      System.out.println("% integer remainder");
      System.out.println("m unary minus");
      System.out.println("q quit");
      System.out.println("h,? this help");

    }
}

以下代码是我用来运行它的驱动程序。

import java.util.*;

public class Calculator {

    public static void main(String[] args){
     String expression, again;
     int result;

     Scanner in = new Scanner(System.in);{

     do
     {  
         RPNCalculator evaluator = new RPNCalculator();
         System.out.println("Please enter at least two integer you want to calculate followed by the operator (e.g. 5 4 + 3 2 1 - + *)");
         System.out.println("Each token must be an integer or an operator (+,-,*,/,p,n,d,r,f,c,%,m,q,h)");
         expression = in.nextLine();

         result = RPNCalculator.performCalculation(expression);
         System.out.println();
         System.out.println("That expression equals " + result);

         System.out.print("Evaluate another expression [Y/N]? ");
         again = in.nextLine();
         System.out.println();
     }
     while (again.equalsIgnoreCase("y"));

     }  
    }
}